The present invention relates to a single crystal of a silicate of a rare earth element, which may be used in, for instance, a scintillator for PET devices, and the scintillator using the same.
A single crystal of a silicate of a rare earth element such as a single crystal of gadolinium silicate has widely been used in a variety of fields such as scintillators and fluorescence materials (phosphors). Such a single crystal of, for instance, gadolinium silicate is prepared from an oxide of a rare earth element such as gadolinium oxide and a silicon oxide such as silicon dioxide as raw materials according to, for instance, the Czochralski method in which a single crystal is grown from a melt containing raw materials. Moreover, such a single crystal of a silicate of a rare earth element is in general doped with an additive such as Ce, which serves as a fluorescent center in the resulting single crystal. It is believed that rare earth elements other than constitutive elements and impurities such as transition metals have an influence on scintillator performances such as fluorescence-attenuation time and therefore, raw materials having a purity of not less than 99.99% by mass such as Gd2O3 and SiO2 from which impure elements have been reduced are used to grow crystals.
When using a conventional single crystal of a silicate of a rare earth element such as that of gadolinium silicate as a scintillator, the luminescence-attenuation curve thereof comprises two components or a rapidly attenuating component (Fast Component) appearing in the range of from 30 to 60 ns and a slowly attenuating component (Slow Component) appearing in the range of from 400 to 600 ns. In this connection, the output ratio (or the existing ratio) of the rapidly attenuating component (Fast Component) to the slowly attenuating component (Slow Component) was found to be on the order of about 70 to 80%: 30 to 20%. For this reason, in case of the scintillator for PET (Positron Emission Computed Tomography Device) whose fluorescence-attenuation time should be reduced, it has been desired to accelerate only the slowly attenuating component (Slow Component) in the luminescence-attenuation curve and to thus reduce the output ratio (or the existing ratio), without impairing other characteristic properties required for such a scintillator. As an example of a means for shortening the fluorescence-attenuation time, there has been known a method in which Ce as a dopant is incorporated into a single crystal for use in the preparation of such a scintillator in an amount higher than that used in the conventional technique.
In a gadolinium silicate single crystal in which the concentration of Ce serving as the fluorescent center is not less than 0.6 mole %, however, the resulting single crystal is dyed in pale yellow. Such coloration is not preferred since it may deteriorate the fluorescent output and the energy resolution and accordingly, it may impair the characteristic properties of the resulting scintillator. This coloration may be caused due to the presence of tetravalent Ce atoms, which do not have any contribution to the luminescence. Accordingly, it would be necessary to devise a measure to reduce the concentration of tetravalent Ce atoms, which may cause the foregoing coloration, while ensuring the reduction of the fluorescence-attenuation time by maintaining a high concentration of Ce, in order to develop a method for simultaneously allowing the shortening of the fluorescence-attenuation time and the maintenance of a high fluorescent output.